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RFC 6436

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Rationale for Update to the IPv6 Flow Label Specification


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Internet Engineering Task Force (IETF)                         S. Amante
Request for Comments: 6436                                       Level 3
Category: Informational                                     B. Carpenter
ISSN: 2070-1721                                        Univ. of Auckland
                                                                S. Jiang
                                                           November 2011

       Rationale for Update to the IPv6 Flow Label Specification


   Various published proposals for use of the IPv6 flow label are
   incompatible with its original specification in RFC 3697.
   Furthermore, very little practical use is made of the flow label,
   partly due to some uncertainties about the correct interpretation of
   the specification.  This document discusses and motivates changes to
   the specification in order to clarify it and to introduce some
   additional flexibility.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Not all documents
   approved by the IESG are a candidate for any level of Internet
   Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at

Page 2 
Copyright Notice

   Copyright (c) 2011 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   ( in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  2
   2.  Impact of Current Specification  . . . . . . . . . . . . . . .  3
   3.  Changes to the Specification . . . . . . . . . . . . . . . . .  6
   4.  Discussion . . . . . . . . . . . . . . . . . . . . . . . . . .  8
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . .  9
   6.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . .  9
   7.  Informative References . . . . . . . . . . . . . . . . . . . . 10
   Appendix A.  Alternative Approaches  . . . . . . . . . . . . . . . 12

1.  Introduction

   The flow label field in the IPv6 header was reserved but left
   Experimental by [RFC2460], which mandates only that "Hosts or routers
   that do not support the functions of the Flow Label field are
   required to set the field to zero when originating a packet, pass the
   field on unchanged when forwarding a packet, and ignore the field
   when receiving a packet."

   The flow label field was normatively specified by [RFC3697].  In
   particular, we quote three rules from that RFC:

   a.  "The Flow Label value set by the source MUST be delivered
       unchanged to the destination node(s)."

   b.  "IPv6 nodes MUST NOT assume any mathematical or other properties
       of the Flow Label values assigned by source nodes."

   c.  "Router performance SHOULD NOT be dependent on the distribution
       of the Flow Label values.  Especially, the Flow Label bits alone
       make poor material for a hash key."

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   Additionally, RFC 3697 does not define the method a host should adopt
   by default to choose the value of the flow label, if no specific
   method is in use.  It was expected that various signaling methods
   might be defined for agreeing on values of the flow label, but no
   such methods have been standardized, except a pre-existing option in
   RSVP [RFC2205].

   The flow label is hardly used in practice in widespread IPv6
   implementations, although some operating systems do set it
   [McGann05].  To some extent, this is due to the main focus being on
   basic deployment of IPv6, but the absence of a default method of
   choosing the flow label value means that most host implementations
   simply set it to zero.  There is also anecdotal evidence that the
   rules quoted above have led to uncertainty about exactly what is
   possible.  Furthermore, various use cases have been proposed that
   infringe one or another of the rules.  None of these proposals has
   been accepted as a standard and in practice there is no significant
   deployment of any mechanism to set the flow label.

   The intention of this document is to explain this situation in more
   detail and to motivate changes to RFC 3697 intended to remove the
   uncertainties and encourage active usage of the flow label.  It does
   not formally update RFC 3697, but it serves as background material
   for [RFC6437].

2.  Impact of Current Specification

   Rule (a) makes it impossible for the routing system to use the flow
   label as any form of dynamic routing tag.  This was a conscious
   choice in the early design of IPv6, and there appears to be no
   practical possibility of revisiting this decision at this stage in
   the deployment of IPv6, which uses conventional routing mechanisms
   like those used for IPv4.  However, this rule also makes it
   impossible to make any use at all of the flow label unless hosts
   choose to set it.  It also forbids clearing the flow label for
   security reasons.

   This last point highlights the security properties, or rather the
   lack thereof, with regards to the flow label.  The flow label field
   is always unprotected as it travels through the network, because
   there is no IPv6 header checksum, and the flow label is not included
   in transport pseudo-header checksums, nor in IPsec checksums.  As a
   result, intentional and malicious changes to its value cannot be
   detected.  Also, it could be used as a covert data channel, since
   apparently pseudo-random flow label values could in fact consist of
   covert data [NIST].  If the flow label were to carry quality-of-
   service semantics, then like the diffserv code point [RFC2474], it
   would not be intrinsically trustworthy across domain boundaries.  As

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   a result, some security specialists believe that flow labels should
   be cleared for safety [LABEL-SEC] [NSA].  These points must be
   considered when discussing the immutability of the flow label across
   domain boundaries.  In fact, the adjective "immutable" is confusing,
   since it implies a property that the flow label field does not
   actually possess.  It has therefore been abandoned as a descriptive
   term in [RFC6437].  It is only used in the present document to
   explain why it has been abandoned.

   Rule (b) appears to forbid any usage in which the bits of the flow
   label are encoded with a specific semantic meaning.  However, the
   words "MUST NOT assume" are to be interpreted precisely - if a router
   knows by configuration or by signaling that the flow label has been
   assigned in a certain way, it can make use of that knowledge.  It is
   not made clear by the rule that there is an implied distinction
   between stateless models (in which there is no signaling, so no
   specific assumption about the meaning of the flow label value can be
   made) and stateful models (in which there is signaling and the router
   has explicit knowledge about the label).

   If the word "alone" is overlooked, rule (c) has sometimes been
   interpreted as forbidding the use of the flow label as part of a hash
   used by load distribution mechanisms.  In this case too, the word
   "alone" needs to be taken into account - a router is allowed to
   combine the flow label value with other data in order to produce a
   uniformly distributed hash.

   Both before and after these rules were laid down, a considerable
   number of proposals for use of the flow label were published that
   seem incompatible with them.  Numerous examples and an analysis are
   presented in [RFC6294].  Those examples propose use cases in which
   some or all of the following apply:

   o  The flow label may be changed by intermediate systems.

   o  It doesn't matter if the flow label is changed, because the
      receiver doesn't use it.

   o  Some or all bits of the flow label are encoded: they have specific
      meanings understood by routers and switches along the path.

   o  The encoding is related to the required quality of service, as
      well as identifying a flow.

   o  The flow label is used to control forwarding or switching in some

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   These proposals require either some form of semantics encoding in the
   bits of the flow label, or the ability for routers to modify the flow
   label, or both.  Thus, they appear to infringe the rules from RFC
   3697 quoted above.

   We can conclude that a considerable number of researchers and
   designers have been stymied by RFC 3697.  On the other hand, some
   other proposals discussed in [RFC6294] appear to be compatible with
   RFC 3697.  Several are based on the originator of a packet choosing a
   pseudo-random flow label for each flow, which is one option suggested
   in RFC 3697.  Thus, we can also conclude that there is a useful role
   for this approach.

   If our goal is for the flow label to be used in practice, the
   conflict between the various approaches creates a dilemma.  There
   appear to be two major options:

   1.  Discourage locally defined and/or stateful use of the flow label.
       Strengthen RFC 3697 to say that hosts should set a label value,
       without necessarily creating state, which would clarify and limit
       its possible uses.  In particular, its use for load distribution
       and balancing would be encouraged.

   2.  Relax the rules to encourage locally defined and/or stateful use
       of the flow label.  This approach would make the flow label
       completely mutable and would exclude use cases depending on
       strict end-to-end immutability.  It would encourage applications
       of a pseudo-random flow label, such as load distribution, on a
       local basis, but it would exclude end-to-end applications.

   There was considerable debate about these options and their variants
   during 2010 - 2011, with a variety of proposals in previous versions
   of this document and in mailing list discussions.  After these
   discussions, there appears to be a view that simplicity should
   prevail, and that complicated proposals such as defining quality-of-
   service semantics in the flow label, or sub-dividing the flow label
   field into smaller sub-fields, will not prove efficient or
   deployable, especially in high-speed routers.  There is also a
   clearly expressed view that using the flow label for various forms of
   stateless load distribution is the best simple application for it.
   At the same time, it is necessary to recognize that the strict
   immutability rule has drawbacks as noted above.

   Even under the rules of RFC 3697, the flow label is intrinsically
   untrustworthy, because modifications en route cannot be detected.
   For this reason, even with the current strict immutability rule,
   downstream nodes cannot rely mathematically on the value being
   unchanged.  In this sense, any use of the flow label must be viewed

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   as an optimization on a best-effort basis; a packet with a changed
   (or zero) flow label value should never cause a hard failure.

   The remainder of this document discusses specific modifications to
   the standard, which are defined normatively in a companion document

3.  Changes to the Specification

   Although RFC 3697 requires that the flow label be delivered
   unchanged, as noted above, it is not included in any transport-layer
   pseudo-header checksums nor in IPsec authentication [RFC4302].  Both
   RFC 2460 and RFC 3697 define the default flow label to be zero.  At
   the time of writing, this is the observed value in an overwhelming
   proportion of IPv6 packets; the most widespread operating systems and
   applications do not set it, and routers do not rely on it.  Thus,
   there is no reason to expect operational difficulties if a careful
   change is made to the rules of RFC 3697.

   In particular, the facts that the label is not checksummed and rarely
   used mean that the "immutability" of the label can be moderated
   without serious operational consequences.

   The purposes of the proposed changes are to remove the uncertainties
   left by RFC 3697, in order to encourage setting of the flow label by
   default, and to enable its generic use.  The proposed generic use is
   to encourage uniformly distributed flow labels that can be used to
   assist load distribution or balancing.  There should be no impact on
   existing IETF specifications other than RFC 3697 and no impact on
   currently operational software and hardware.

   A secondary purpose is to allow changes to the flow label in a
   limited way, to allow hosts that do not set the flow label to benefit
   from it nevertheless.  The fact that the flow label may in practice
   be changed en route is also reflected in the reformulation of the

   A general description of the changes follows.  The normative text is
   to be found in [RFC6437].

   The definition of a flow is subtly changed from RFC 3697 to allow any
   node, not just the source node, to set the flow label value.
   However, it is recommended that sources should set a uniformly
   distributed flow label value in all flows, replacing the less precise
   recommendation made in Section 3 of RFC 3697.  Both stateful and
   stateless methods of assigning a uniformly distributed value could be

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   Flow label values should be chosen such that their bits exhibit a
   high degree of variability, thus making them suitable for use as part
   of the input to a hash function used in a load distribution scheme.
   At the same time, third parties should have a low probability of
   guessing the next value that a source of flow labels will choose.

   In statistics, a discrete uniform distribution is defined as a
   probability distribution in which each value in a given range of
   equally spaced values (such as a sequence of integers) is equally
   likely to be chosen as the next value.  The values in such a
   distribution exhibit both variability and unguessability.  Thus, an
   approximation to a discrete uniform distribution is preferable as the
   source of flow label values.  In contrast, an implementation in which
   flow labels are assigned sequentially is definitely not recommended,
   to avoid guessability.

   In practice, it is expected that a uniform distribution of flow label
   values will be approximated by use of a hash function or a pseudo-
   random number generator.  Either approach will produce values that
   will appear pseudo-random to an external observer.

   Section 3 of RFC 3697 allows nodes to participate in an unspecified
   stateful method of flow state establishment.  The changes do not
   remove that option, but clarify that stateless models are also
   possible and are the recommended default. The specific text on
   requirements for stateful models has been reduced to a bare minimum
   requirement that they do not interfere with the stateless model.  To
   enable stateless load distribution at any point in the Internet, a
   node using a stateful model should never send packets whose flow
   label values do not conform to a uniform distribution.

   The main novelty is that a forwarding node (typically a first-hop or
   ingress router) may set the flow label value if the source has not
   done so, according to the same recommendations that apply to the
   source.  This might place a considerable processing load on ingress
   routers that choose to do so, even if they adopted a stateless method
   of flow identification and label assignment.

   The value of the flow label, once it has been set, must not be
   changed.  However, some qualifications are placed on this rule, to
   allow for the fact that the flow label is an unprotected field and
   might be misused.  No Internet-wide mechanism can depend
   mathematically on immutable flow labels.  The new rules require that
   flow labels exported to the Internet should always be either zero or
   uniformly distributed, but even this cannot be relied on
   mathematically.  Use cases need to be robust against non-conforming

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   flow label values.  This will also enhance compatibility with any
   legacy hosts that set the flow label according to RFC 2460 and RFC

   A complication that led to much discussion is the possibility that
   hosts inside a particular network domain might use a stateful method
   of setting the flow label, and that packets bearing stateful labels
   might then erroneously escape the domain and be received by nodes
   performing stateless processing, such as load balancing.  This might
   result in undesirable operational implications (e.g., congestion,
   reordering) for not only the inappropriately flow-labeled packets,
   but also well-behaved flow-labeled packets, during forwarding at
   various intermediate devices.  It was suggested that border routers
   might "correct" this problem by overwriting such labels in packets
   leaving the domain.  However, neither domain border egress routers
   nor intermediate routers/devices (using a flow label, for example, as
   a part of an input key for a load-distribution hash) can determine by
   inspection that a value is not part of a uniform distribution.  Thus,
   there is no way that such values can be detected and "corrected".
   Therefore, the recommendation to choose flow labels from a uniform
   distribution also applies to stateful schemes.

4.  Discussion

   The following are some practical consequences of the above changes:

   o  Sending hosts that are not updated will in practice continue to
      send all-zero labels.  If there is no label-setting router along
      the path taken by a packet, the label will be delivered as zero.

   o  Sending hosts conforming to the new specification will by default
      choose uniformly distributed labels between 1 and 0xFFFFF.

   o  Sending hosts may continue to send all-zero labels, in which case
      an ingress router may set uniformly distributed labels between 1
      and 0xFFFFF.

   o  The flow label is no longer unrealistically asserted to be
      strictly immutable; it is recognized that it may, incorrectly, be
      changed en route.  In some circumstances, this will break end-to-
      end usage, e.g., potential detection of third-party spoofing
      attacks [LABEL-SEC].

   o  The expected default usage of the flow label is some form of
      stateless load distribution, such as the ECMP/LAG usage defined in

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   o  If the new rules are followed, all IPv6 traffic flows on the
      Internet should have zero or uniformly distributed flow label

   From an operational viewpoint, existing IPv6 hosts that set a default
   (zero) flow label value and ignore the flow label on receipt will be
   unaffected by implementations of the new specification.  In general,
   it is assumed that hosts will ignore the value of the flow label on
   receipt; it cannot be relied on as an end-to-end signal.  However,
   this doesn't apply if a cryptographically generated label is being
   used to detect attackers [LABEL-SEC].

   Similarly, routers that ignore the flow label will be unaffected by
   implementations of the specification.

   Hosts that set a default (zero) flow label but are in a domain where
   routers set a label as recommended in Section 3 will benefit from
   whatever flow label handling is used on the path.

   Hosts and routers that adopt the recommended mechanism will enhance
   the performance of any load balancing devices that include the flow
   label in the hash used to select a particular path or server, even
   when packets leave the local domain.

5.  Security Considerations

   See [RFC6437] and [LABEL-SEC] for full discussion.  Some useful
   remarks are in [Partridge].

6.  Acknowledgements

   The authors are grateful to Qinwen Hu for general discussion about
   the flow label and for his work in searching the literature.
   Valuable comments and contributions were made by Ran Atkinson, Fred
   Baker, Steve Blake, Remi Despres, Alan Ford, Fernando Gont, Brian
   Haberman, Tony Hain, Joel Halpern, Chris Morrow, Thomas Narten, Pekka
   Savola, Mark Smith, Pascal Thubert, Iljitsch van Beijnum, and other
   participants in the 6man working group.

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7.  Informative References

   [FLOWSWITCH]  Beckman, M., "IPv6 Dynamic Flow Label Switching (FLS)",
                 Work in Progress, March 2007.

   [LABEL-SEC]   Gont, F., "Security Assessment of the IPv6 Flow Label",
                 Work in Progress, November 2010.

   [McGann05]    McGann, O. and D. Malone, "Flow Label Filtering
                 Feasibility", European Conference on Computer Network
                 Defence , 2005.

   [NIST]        Frankel, S., Graveman, R., Pearce, J., and M. Rooks,
                 "Guidelines for the Secure Deployment of IPv6",
                 National Institute of Standards and Technology Special
                 Publication 800-119, 2010, <

   [NSA]         Potyraj, C., "Firewall Design Considerations for IPv6",
                 National Security Agency I733-041R-2007, 2007,

   [Partridge]   Partridge, C., Arsenault, A., and S. Kent, "Information
                 Assurance and the Transition to IP Version 6 (IPv6)",
                 Military Communications Conference (MILCOM 2007)  ,
                 2007, <

   [RFC2205]     Braden, B., Zhang, L., Berson, S., Herzog, S., and S.
                 Jamin, "Resource ReSerVation Protocol (RSVP) -- Version
                 1 Functional Specification", RFC 2205, September 1997.

   [RFC2460]     Deering, S. and R. Hinden, "Internet Protocol, Version
                 6 (IPv6) Specification", RFC 2460, December 1998.

   [RFC2474]     Nichols, K., Blake, S., Baker, F., and D. Black,
                 "Definition of the Differentiated Services Field (DS
                 Field) in the IPv4 and IPv6 Headers", RFC 2474,
                 December 1998.

   [RFC3697]     Rajahalme, J., Conta, A., Carpenter, B., and S.
                 Deering, "IPv6 Flow Label Specification", RFC 3697,
                 March 2004.

   [RFC4302]     Kent, S., "IP Authentication Header", RFC 4302,
                 December 2005.

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   [RFC6294]     Hu, Q. and B. Carpenter, "Survey of Proposed Use Cases
                 for the IPv6 Flow Label", RFC 6294, June 2011.

   [RFC6437]     Amante, S., Carpenter, B., Jiang, S., and J. Rajahalme,
                 "IPv6 Flow Label Specification", RFC 6437, November

   [RFC6438]     Carpenter, B. and S. Amante, "Using the IPv6 Flow Label
                 for Equal Cost Multipath Routing and Link Aggregation
                 in Tunnels", RFC 6438, November 2011.

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Appendix A.  Alternative Approaches

   A model was discussed in an earlier version of this document which
   defined a notion of 'flow label domain' analogous to a differentiated
   services domain [RFC2474].  This model would have encouraged local
   usage of the flow label as an alternative to any form of generic use,
   but it required complex rules for the behavior of domain boundary
   routers, and proved controversial in discussion.

   Two even more complex alternative approaches were also considered and

   The first was to distinguish locally significant flow labels from
   those conforming to RFC 3697 by setting or clearing the most
   significant bit (MSB) of the flow label.  This led to quite
   complicated rules, seems impossible to make fully self-consistent,
   and was not considered practical.

   The second was to use a specific differentiated services code point
   (DSCP) [RFC2474] in the Traffic Class octet instead of the MSB of the
   flow label itself, to flag a locally defined behavior.  A more
   elaborate version of this was proposed in [FLOWSWITCH].  There are
   two issues with that approach.  One is that DSCP values are
   themselves only locally significant, inconsistent with the end-to-end
   nature of the original flow label definition.  Secondly, it seems
   unwise to meld the semantics of differentiated services, which are
   currently deployed, with the unknown future semantics of flow label
   usage.  However, this approach, while not recommended, does not
   appear to violate any basic principles if applied strictly within a
   single differentiated services domain.

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Authors' Addresses

   Shane Amante
   Level 3 Communications, LLC
   1025 Eldorado Blvd
   Broomfield, CO  80021


   Brian Carpenter
   Department of Computer Science
   University of Auckland
   PB 92019
   Auckland,   1142
   New Zealand


   Sheng Jiang
   Huawei Technologies Co., Ltd
   Q14, Huawei Campus
   No.156 Beiqing Road
   Hai-Dian District, Beijing  100095
   P.R. China